The object of the present research project is to identify in the rat the precursors of thoracic duct small lymphocytes and to determine the stages of development and anatomical sites in which small lymphocytes or their precursors acquire the capacity to initiate an immunological response. Central to this study is the role of specific cell surface "recognition sites" in determining the unique migration pathways of lymphocytes and their precursors; and the function of the thymus in generation and/or maturation of immunologically competent lymphocytes.